Axial and radial development of the hot electron distribution in a helicon plasma source, measured by a retarding field energy analyzer (RFEA)

Abstract

The information about the electron population of a helicon source plasma that expands along a magnetic nozzle is important for understanding the plasma acceleration across the potential drop that forms in the nozzle. The electrons need an energy higher than the potential drop to escape from the source. At these energies the signal of a Langmuir probe is less accurate. An inverted RFEA measures the high-energy tail of the electrons. To reach the probe, they must have energies above the plasma potential VP, which can vary over the region of the measurement. By constructing a full distribution by applying the electron temperature Te obtained from the electron IV-curve and the VP obtained from the ion collecting RFEA or an emissive probe, a density measure of the hot electron distribution independent of VP can be obtained. The variation of the high-energy tail of the EEDF in both radial and axial directions, in the two different cases of 1) a purely expanding magnetic field nozzle, and 2) a more constricted one by applying current in a third, downstream coil was investigated. The electron densities and temperatures from the source are then compared to two analytic models of the downstream development of the electron density. The first model considers the development for a pure Boltzmann distribution while the second model takes an additional magnetic field expansion into account. A good match between the measured densities and the second model was found for both configurations. The RFEA probe also allows for directional measurement of the electron current to the probe. This property is used to compare the densities from the downstream and upstream directions, showing a much lower contribution of downstream electrons into the source for a purely expanding magnetic field in comparison to the confined magnetic field configuration.

Effects of magnetic nozzle strength and orientation on radio-frequency plasma expansion

“…Numerous experiments and simulations have reported the formation of a hollow density structure, also called highdensity conics, in the diffusion region of an RF plasma expanding in a symmetric magnetic nozzle [13][14][15][16][17][18][19][20][21][22][23][24]. A common feature in the literature is the concomitance of the conics with the most radial streamlines leaving the source.…”

“…A common feature in the literature is the concomitance of the conics with the most radial streamlines leaving the source. The generation of these high-density conics has been attributed to highenergy electrons created a few skin depths under the antenna that escape the plasma source along the last radial field lines leaving the glass tube and causing off-axis local ionisation [15,16,19,22,24,25]. Takahashi et al [16] have shown the presence of a high electron temperature region all along the field lines which intersect the antenna and can expand downstream.…”